An automatic transmission includes two or more friction engagement elements (a clutch, a brake) whose engagement states can be hydraulically switched. The automatic transmission switches among power transmission paths by causing one friction engagement element to be engaged while another friction engagement element is disengaged, thereby realizing shift and switching between forward and reverse movements.
Some automatic transmissions perform neutral idle control for improving fuel economy while a vehicle is stopped. According to this control, while the vehicle is stopped, the engine load is decreased by placing a friction engagement element used in starting the vehicle in a state that immediately precedes the engagement, thus setting an engagement capacity thereof to zero.
In order to realize a smooth start, shock-reduced shift, and neutral idle control, it is important to correctly discover a hydraulic pressure (engagement start pressure) at which a friction engagement element starts to hold a transmission capacity (transmittable torque).
In view of this, automatic transmissions learn an engagement start pressure of a friction engagement element while a vehicle is stopped (JP 2008-106814A). In the course of learning, a hydraulic pressure supplied to a friction engagement element targeted for learning is increased and decreased. An engagement start pressure of this friction engagement element is learnt based on a resultant change in a rotation speed of a rotation element of the automatic transmission.